1. Field of the Disclosure
The present invention relates generally to a method and apparatus for providing pressure compensation for a pressure drop due to dynamic flow in a pressure support system.
2. Description of the Related Art
Positive pressure ventilatory modes of operation are commonly used in devices that provide respiratory assistance, such as continuous positive airway pressure (CPAP) devices. The positive pressure devices are typically programmed by a physician to provide a specified pressure and often different inspiratory and expiratory pressures. Alternatively, the positive pressure devices can be used in a volume control mode, in which a physician programs the volume to be delivered on a given breath.
In order to provide the prescribed pressure or volume, the pressure device typically measures pressure and flow near the outlet of the device and then estimates the pressure and volume that is being provided to the airways of the patient. The patient circuit tubing has characteristics that affect the pressure and flow delivered to the patient. The device typically estimates such effects on the pressure and flow to provide the proper amount of pressure.
Compensation for patient tubing is known to those skilled in the art. For example, there are devices that compensate for the flow resistance of the patient circuit tubing, such as using “automatic tube compensation,” where the pressure drop over the patient circuit is estimated to be proportional to the square of the flow rate and a proportionality constant is defined by the user input. There are also devices that compensate for the shortfall of the volume delivered to the patient due to pneumatic compliance.
The amount of pressure drop over the patient circuit is estimated so that an accurate or proper amount of pressure may be provided to the patient. Regulatory standards require positive pressure devices to meet certain accuracy standards under dynamic flow conditions. While typical pressure support devices may be able to provide pressure at a level that is sufficiently comfortable for the patient and can meet regulatory requirements without compensating for the pressure drop due to dynamic flow, the tubing typically used heretofore has been of a relatively large diameter. If smaller diameter tubing were to be used, the pressure drop due to the dynamic flow in the smaller tubing would cause the pressure support systems to have more undesirable pressure “swings.” Consequently, the pressure provided to the patient may be less comfortable and may be unable to meet regulatory standards.
For example, the inductance of air in a tube, L, is defined as:
                              L          =                                                    ρ                air                            ·                              l                tube                                                                    A                tube                            ·              g                                      ,                            Eq        .                                  ⁢        1.1            where:L is the inductance of the tube in cmH2O/(liters/sec2),ρair is the density of the air, grams/liter,ltube is the length of the tube in cm,Atube is the cross-sectional area of the tube, cm2, andg is the gravitational constant, 981 cm/sec2.
Therefore, a nominal inductance value might be 0.0775 cmH2O/(liters/sec2) for a standard-sized tube having a length of 6 ft. and an internal diameter of 22 mm used for non-invasive pressure therapy. During normal respiration, the total pressure swing caused by the inductive effect may only be on the order of 0.2-0.3 cmH2O, peak to peak. This small, transient error is often not enough for the patient to detect. If the diameter of a six foot tubing were to be as small as 15 mm, 11 mm, or perhaps even smaller, problems may result. For example, in the case of 11 mm tubing, the inductance value may be as high as 0.310 cmH2O/(liters/sec2), and the peak-to-peak pressure error could be up to 1.2cmH2O, which could cause discomfort to the patient.
Accordingly, it is an object of the present invention to provide a pressure support system that overcomes the shortcomings of conventional pressure support system. This object is achieved according to one embodiment of the present invention by providing a pressure support system t for delivering a flow of gas to an airway of a patient that include a gas flow generator that generates a flow of gas and a patient circuit. The patient circuit is coupled to the gas flow generator and comprises a conduit adapted to communicate the flow of gas to an airway of a patient. The system also includes a sensor associated with the gas flow generator and/or the patient circuit and is configured to measure the flow of gas in the patient circuit and generate flow signals based on the measured flow. The system further includes a controller operatively connected with the gas flow generator and the sensor. The controller is configured to control a pressure of the flow of gas provided to the patient. The controller receives the flow signals from the sensor and determines a rate of change in the flow of the gas provided to the patient. The controller modifies or compensates the pressure of the flow of gas provided to the patient if the rate of change in the flow of gas provided to the patient exceeds a threshold amount.